Global Navigation Satellite Systems (GNSS) include the Global Positioning System (GPS), the Glonass system, the proposed Galileo system, the proposed Compass system, and others. Each GPS satellite transmits continuously using two radio frequencies in the L-band, referred to as L1 and L2, at respective frequencies of 1575.41 MHz and 1227.60 MHz. Two signals are transmitted on L1, one for civil users and the other for users authorized by the United States Department of Defense (DoD). One signal is transmitted on L2, intended only for DoD-authorized users. Each GPS signal has a carrier at the L1 and L2 frequency, a pseudo-random number (PEN) code, and satellite navigation data. Two different PRN codes are transmitted by each satellite: a coarse acquisition (C/A) code and a precision (P/Y) code which is encrypted for DoD-authorized users. Each C/A code is a unique sequence of 1023 bits, which is repeated each millisecond. Other GNSS systems likewise have satellites which transmit multiple signals on multiple carrier frequencies.
FIG. 1 schematically illustrates a typical prior-art scenario to determine the position of a mobile receiver (rover). Rover 100 receives GPS signals from any number of satellites in view, such as SV1, SV2, and SVM, shown respectively at 110, 120 and 130. The signals pass through the earth's ionosphere 140 and through the earth's troposphere 150. Each signal has two frequencies, L1 and L2. Receiver 100 determines from the signals respective pseudo-ranges, PR1, PR2, . . . , PRM, to each of the satellites. Pseudo-range determinations are distorted by variations in the signal paths which result from passage of the signals through the ionosphere 140 and the troposphere 150, and from multipath effects, as indicated schematically at 160.
Pseudo-range can be determined using the C/A code with an error of about one meter, a civil receiver not using the military-only P/Y code determines rover position with an error in the range of meters. However, the phases of the L1 and L2 carriers can be measured with an accuracy of 0.01-0.05 cycles (corresponding to pseudo-range errors of 2 mm to 1 cm), allowing relative position of the rover to be estimated wish errors in the range of millimeters to centimeters. Accurately measuring the phase of the L1 and L2 earners requires a good knowledge of the effect of the ionosphere and the troposphere for all observation times.
Relative positioning allows common-mode errors to be mitigated by differencing the observations of the rover with observations of a reference station at a known location near the rover, e.g., within 50-100 km. The reference station observations can be collected at a physical base station or estimated from observations of a network of reference stations. See for example U.S. Pat. No. 5,477,458 “Network for Carrier Phase Differential GPS Corrections” and U.S. Pat. No. 5,899,957 “Carrier Phase Differential GPS Collections Network.”
Precise point positioning (PPP), also called absolute positioning, uses a single GNSS receiver together with precise satellite orbit and clock data to reduce satellite-related error sources. A dual-frequency receiver can remove the first-order effect of the ionosphere for position solutions of centimeters to decimeters. The utility of PPP is limited by the need to wait longer than desired for the float position solution to converge to centimeter accuracy. And unlike relative positioning techniques in which common-mode errors are eliminated by differencing of observations, PPP processing uses undifferenced carrier-phase observations so that the ambiguity terms are corrupted by satellite and receiver phase biases. Methods have been proposed for integer ambiguity resolution in PPP processing. See, for example, Y. Gao et ah, GNSS Solutions: Precise Point Positioning and Its Challenges, Inside GNSS, November/December 2006, pp. 16-18. See also U.S. Provisional Application for Patent No. 61/277,184 filed 19 Sep. 2009 (TNL A-2585P).
Improved GNSS processing methods and apparatus are desired, especially to achieve faster convergence to a solution, improved accuracy and/or greater availability.